1. Field of the Invention
The present invention relates to an optical module that is used in optical communication technology, optical transmission technology, and optical information recording technology, and in particular relates to an optical coupling structure that optically couples an optical semiconductor element and an optical transmission path in an optical module.
2. Description of the Related Art
An optical module is equipped with an optical semiconductor element that is mounted on a substrate, and an optical transmission path that is arranged so that the optical axis is parallel with the substrate.
Conventionally, in this type of optical module, in order to optically couple the light receiving/emitting portion of the optical semiconductor element and an end portion of the optical transmission path, the structure as shown in FIG. 16 is generally used. In this structure, by combining a condenser lens 102 that is arranged on an optical semiconductor element 101, and an optical path conversion mirror 103, an optical transmission path 104 (in particular, a core 105 thereof) and the optical semiconductor element 101 are optically connected (optically coupled).
In such a structure, it is necessary for the refractive index of the condenser lens 102 and the reflectance of the optical conversion mirror 103 to be adjusted to desired values. In addition, the number of parts required for optical coupling increases. Moreover, it is necessary to precisely match the respective positional relations of the optical semiconductor element 101, the condenser lens 102, the optical conversion mirror 103, and the optical transmission path 104. For that reason, the cost of components, and the cost of the work related to assembly are high, becoming the principal cause of cost increases.
In order to reduce the manufacturing cost of such an optical module and to provide an optical module at a lower cost, an example of optical device is proposed in Japanese Unexamined Patent Application, First Publication No. 2003-167175, that includes a guide groove in the surface of an optical semiconductor chip mounted substrate, and a tapered surface that is positioned over the optical axis of an optical fiber that is mounted in this guide groove, with a mirror formed in the tapered surface.
In Japanese Patent No. 2985791, a coupling structure of an optical waveguide and a light receiving element is proposed that includes a substrate in which is formed a V-groove having a reflecting surface that is obliquely formed at a position that faces the end portion of an optical waveguide, a refractive index matching agent that fills the space between the end portion of the optical waveguide and the reflecting surface and that has nearly the same refractive index as the core of the optical waveguide, and a light receiving element that receives the outgoing light that is reflected by the reflecting surface.
In Japanese Unexamined Patent Application, First Publication No. H09-197196, a method for joining optical components is disclosed that can achieve highly accurate and simple connections between optical components in an optical transreceiver module. In this method, the optical fiber and the light receiving/emitting element, which are arranged so that the optical axes substantially coincide, are pressure bonded with an uncured transparent resin composition therebetween, and after pulling back the optical fiber and extending the uncured transparent resin composition (photo-curable, thermosetting, or thermoplastic), the extended transparent resin composition is cured.
Japanese Unexamined Patent Application, First Publication No. 2000-269584 proposes a semiconductor laser device in which a semiconductor laser element, a monitor photo diode, and an optical fiber are sealed in a transparent resin, and the rearward output light of the semiconductor laser element is reflected at the interface between the transparent resin and air, and enters the monitor photo diode.
However, in the optical device of Patent Document 1, preparation of a die for forming the guide groove and tapered surface, and the step of forming the mirror with a tapered surface are necessary. As a result, there is the problem of the manufacturing cost increasing. Moreover, in an optical semiconductor chip mounted substrate having a guide groove, since it is necessary to perform flip chip bonding with the optical semiconductor element facing down, the line length from the optical semiconductor element to the integrated circuit that requires wire bonding (for example, between the light receiving element and the IC for amplification) becomes longer. As a result, the problem arises of noise easily containing. Moreover, since it is difficult to perform appearance inspection of the optical semiconductor element that is flip-chip bonded after mounting, it is difficult to discover poor connections thereof.
In the optical coupling structure of Patent Document 2, since it is necessary to form a V groove that has a tapered surface at a position facing the optical fiber that is extremely fine, form a total reflective mirror on that tapered surface, and moreover fill a refractive index matching agent between the end portion of the optical waveguide and the reflective surface, the number of manufacturing steps increases. As a result, there is the problem of the manufacturing cost increasing.
In the method for joining optical components of Patent Document 3, since it is necessary to arrange the optical axes of the optical fiber and the light receiving/emitting element so as to substantially coincide, it can only be applied in the case of both optical axes being coaxial. For that reason, in the case of for example, both optical axes being mutually perpendicular or the like, when attempting to convert an optical path, the problem arises of this method being inapplicable.
Since the semiconductor laser device of Patent Document 4 is for monitoring the rear output light of a semiconductor laser element with a large aperture angle, it can be used even if the efficiency of the optical coupling is low. However, in the case of making an optical signal incident on the light receiving element from an optical fiber with a small aperture angle, or making it incident on an optical fiber from a light emitting element, when the coupling efficiency is low, there is the problem of ensuring the reliability of transmission of the optical signal being difficult. Further, the position and shape of the resin interface that serves as the reflecting surface is considered to depend on the quantity of the applied transparent resin and the shape of a step of the substrate. For that reason, when forming a step in a substrate, since it is necessary to design and manufacture the position and shape of the resin interface in accordance with the shape and size of the step, the manufacturing cost rises. In addition, a reflective surface must exist at the intersection point where the optical axis that is perpendicular to the semiconductor laser element and the optical axis that is perpendicular to the monitor photo diode element intersect. However, forming the resin so that the interface of the resins is positioned just at the position of that intersection point is not easy.
The present invention was achieved in view of the above circumstances, and has an object of providing an optical coupling structure of an optical semiconductor element and an optical transmission path in an optical module that includes an optical coupling portion that can be manufactured at low cost and can transmit an optical signal with higher efficiency, with the optical axis of the optical transmission path and the optical axis of the light receiving/emitting portion in a positional relation forming a predetermined angle.